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polyvinyl alcohol composite films
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CCLET 3760 1–5 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet 1 2 3 4 5 6 7
Original article
Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films Q1 Ji-Xi
Guo, Ming-Xi Guo, Dian-Zeng Jia *, Yin-Hua Li
Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 May 2016 Received in revised form 7 June 2016 Accepted 21 June 2016 Available online xxx
Novel photochromic composite films have been successfully fabricated by dispersing pyrazolone derivative: 1,3-diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (1a) into hydrosol of polyvinyl alcohol (PVA). The microstructure, photochromic behaviors and thermal bleaching properties were investigated by Raman spectroscopy, X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and ultraviolet–visible absorption spectroscopy (UV–vis). The results showed that 1a was not only blended but also well dispersed in the PVA polymer films with a suitable content of chromophore. Upon UV light irradiation, the composite films gradually changed from colorless to yellow and recovered fully to the initial state upon thermal bleaching. The time constants of photochromic reactions were almost the same as those of 1a observed in their crystalline state, indicating that the photochromic phenomenon is barely disturbed by the polymer matrix. ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Pyrazolone Polyvinyl alcohol Composite film Photochromism Information storage
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1. Introduction
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Photochromic materials have attracted considerable attention due to their special structures, properties and various applications [1–5]. Pyrazolone derivatives, a new class of organic photochromic materials, have been proven to be promising and important photochromic compounds due to their high sensitivity and excellent fatigue resistance in the solid state [6–9]. Photochromic materials, which display photochromic reactions in the solid state, are fundamental in developing their applications, such as optical information storage [10–12], optical switches [13–16], anticounterfeiting [17,18] and ultraviolet protection [19]. However, in most cases, the photochromic molecules need to be embedded in complex environments such as bulk polymer materials or thin films in order to translate the molecular response into the desired macroscopic effects [20,21]. Therefore, the functional composite films [22,23], which possess reversible photochromism with distinguished color changes, have become a hot topic in material research [24–26]. Pyrazolone derivatives showed no photochromic reaction in solution and amorphous state for their special
* Corresponding author. E-mail address: [email protected] (D.-Z. Jia).
mechanism with inter- and intramolecular proton transfer [27] and common methods for preparing composite films by dissolving chromophores in solvents are not suitable for this system. Therefore, developing of composite films containing photochromic pyrazolones will be a challenge for extending their application. In previous studies, we have reported a composite film based on dispersing the pyrazolone powders in hydroxypropyl methyl cellulose (HPMC) [27]. The composite films showed a reversible photochromic reaction under UV irradiation and thermal bleaching by heating. Unfortunately, the transparency of the composite films is poor. In this work, we have successfully fabricated a thin composite film with 1,3-diphenyl-4-(3-chlorobenzal)-5-hydro-xypyrazole 4phenylse-micarbazone (1a) and polyvinyl alcohol (PVA) in aqueous solution to avoid inactivation of pyrazolones, as shown in Scheme 1. The choice of PVA is vital for the preparation of the composite films not only due to its good film-forming ability, optical transparency and water solubility, but also due to its surfactant effect that enables the distribution of 1a in water solutions. The fabricated composite films exhibited reversible photoisomerization reactions under alternating UV light irradiation and heating as the same as pure 1a observed in the crystalline state. The optical contrast can be easily modified by changing the composite ratio of 1a and PVA. Moreover, the color of 1a in film can be easier tuned by UV stimuli and fades faster than that of 1a in the crystalline state.
http://dx.doi.org/10.1016/j.cclet.2016.06.037 1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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CCLET 3760 1–5 J.-X. Guo et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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Scheme 1. Photochronmic reaction of pyrazolone 1, chemical structure of PVA and photochromic composite film.
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2. Experimental
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The powder X-ray diffraction (XRD) data of the material were collected on a Bruker D8 Advance X-ray diffractometer. Raman spectra were recorded on a Bruker Senterra (R200-L) raman spectrometer. The field emission scanning electron microscopy (FE-SEM) images were obtained on a Hitachi S-4800 scanning electron microscope. UV–visible absorption spectra were studied using a Hitachi U-3010 spectrometer equipped with an integrating sphere accessory and temperature-controlled accessory. A 15 W lamp in ZF-8 Ultraviolet Analysis Instrument with 365 nm excitation light (1.5 W cm 2 on the sample) and a TECH XT-5 melting point apparatus were used as sources for photocoloration and thermal bleaching, respectively. The distance between the sample and light source was 15 cm. Materials: 1,3-Diphenyl-5-pyrazolone was synthesized according to the literature method [28], 3-chlorobenzoylchloride and 4-phenylsemicarbazide were purchased from the J&K Company. PVA powders were purchased from the Aladdin Company. Other materials were purchased from commercial sources, and the solvents were purified with standard procedures. Synthesis of photochromic pyrazolone derivative: 1,3-Diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenyl-semicarbazone (1a) was synthesized through reaction of 1,3-diphenyl-4(3-chlorobenzal)-5-hydroxypyrazole [29] and 4-phenyl-semicarbazone in the presence of acetic acid in ethanol at 80 8C according to the literature [30]. Preparation of 1a/PVA composite films: The 1a/PVA composite films were prepared by the solvent evaporation method. PVA powders (1.5 g) were added to deionized water (10 mL) with vigorous stirring at 80 8C, which provided a viscous mixture. In dark room, the powder 1a (0.015 g) was slowly added to the above solution with continuous stirring for 3 h at room temperature and then left in the dark until the bubbles disappeared. The mixed solution, 1a in the PVA hydrosol with content of 1 wt%, was cast on a smooth glass groove and dried at ambient temperature in the dark for 10 h. Then the composite film was peeled off from the glass substrates. The fabricated composite film (1 wt%) was placed in the dark to prevent the occurrence of photochemical reactions. For comparison, a pure PVA film without 1a addition and 0.5 wt%, 2 wt% composite films were also fabricated following the same procedure. Of note, powders of 1a cannot be dissolved but only dispersed in water.
Fig. 1. Raman spectra of (a) pure PVA film, (b) pure 1a and the different contents of 1a in PVA film (c: 0.5 wt%, d: 1 wt%, e: 2 wt%).
3. Results and discussion
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3.1. Morphology and structure of 1a/PVA composite films
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Raman spectroscopy is a useful tool in the characterization of successful incorporation of 1a into the polymer matrix. The Raman spectra of the pure PVA film, pure 1a and different contents of 1a in PVA film are shown in Fig. 1. It can be clearly seen that the 1a/PVA composite film exhibits the characteristic vibration peaks of 1a at 1605, 1582 and 1000 cm 1, and these peaks are enhanced with increased contents of 1a in film. Meanwhile, the characteristic bands of PVA also appeared to verify the presence of PVA in the composite film. These results demonstrate that the 1a powders dispersed in the polymer films without any structural alteration.
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Fig. 2. XRD of (a) pure PVA film, (b) pure 1a and the different contents of 1a in PVA film (c: 0.5 wt%, d: 1 wt%, e: 2 wt%).
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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CCLET 3760 1–5 J.-X. Guo et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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Fig. 3. FE-SEM of (a) pure PVA film, insert: pure 1a, and the different contents of 1a in PVA film (b: 0.5 wt%, c: 1 wt%, d: 2 wt%).
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The morphology of materials was characterized by XRD. From Fig. 2, we found that the powders of 1a exhibited the characteristic diffraction peaks at 5.9o, 15.6o, 17.2o, 19.5o and 35.3o (2u), respectively. The lines were sharpened, which demonstrated that the powders of 1a have highly crystalline structures. But the pure PVA film showed an amorphous morphology and presented a broad diffraction peak at 19.7o (2u). We further found that the characteristic diffraction peaks at 5.9o (2u) of 1a in composite films were enhanced with increased contents of 1a in films, and the broad diffraction peak at 19.7o (2u) of PVA still existed in the composite film. These results also provided strong evidence for well dispersing of 1a in the PVA film. The state of 1a in polymer film was further evidenced by FESEM. Based on the above analysis, we know that 1a powders were dispersed into the polymer matrix without any structural alteration. As shown in Fig. 3, the surface of the pure PVA film was very smooth. However, the surface of composite films with different contents of 1a dispersed in PVA matrix became rougher, which showed that rod of 1a was embedded and dispersed in the polymer matrix.
3.2. Photochromic reactions of 1a/PVA composite films
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The powders of 1a can undergo a reversible photochromism reaction upon the alternating treatments of irradiation with UV and heating accompanying with color changes between white and yellow [30]. The absorption spectra of 1a in the crystalline state induced by photoirradiation at room temperature are shown in Fig. 4a. However, the pure PVA film does not respond to UV light. The composite films with different contents of 1a show similar photochromic phenomenon compared with that of the 1a powders (Fig. 4b, c, d). Upon irradiation with 365 nm light, new broad bands in the range of 350–520 nm appeared for composite films with different contents of 1a and their intensities increased with prolonging UV light irradiation. The maximum absorption wavelength of these composite films with different contents of 1a in the PVA matrix was observed at 410 nm, and the ratios of absorbance contrast between the photo-stationary state and initial state were about 3:1 (0.5 wt%), 4:1 (1 wt%) and 5:1 (2 wt%), respectively. These differences are attributed to the different contents of 1a embedded in PVA matrix. The photochromic
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Fig. 4. UV–vis spectra of 1a/PVA composite films (a: pure 1a, b: 0.5 wt%, c: 1 wt%, d: 2 wt%) with different irradiation times at room temperature.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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CCLET 3760 1–5 J.-X. Guo et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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Fig. 5. Fading kinetics of 1b (a) and 1b (1 wt%) in PVA film (c) at different temperatures. (b) and (d) photocoloration-thermalbleaching cycles upon alternating irradiation with UV light and heating for 1a and 1a in PVA film, respectively.
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mechanism of 1a embedded in PVA film is the same as that of 1a in powders. We further investigated the kinetic of 1a powders and the different contents of 1a in PVA films by monitoring their maximum absorbance changes as a function of irradiation time (Fig. S1 in Supporting information). The first-order kinetics was observed by fitting the experimental data to the equation -kt = 1n[(A-A0)/(AAt)], and the rate constants of 1a and 1a in the PVA matrix obtained from the slope were k1a = 0.0277 min 1, k0.5 wt% = 0.0342 min 1, k1 1 and k2 wt% = 0.026 min 1, respectively. We wt% = 0.0285 min found that the composite films with different contents of 1a in the PVA matrix had different coloration rates, the coloration rate turning slower with increased content of 1a powders.
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3.3. Thermal bleaching reactivity
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In order to understand the impact of microenvironment on the composite film, the thermal bleaching of the photochromic compound 1b and 1b in PVA matrix were investigated at different temperatures (Fig. 5a,c). It was found that 1b and 1b/PVA (1 wt%) film could fade at a high speed at elevated temperatures. But the fading speed of composite films was faster than that of 1b at the same temperature. Sample 1b needed 900 s to be completely bleached; however, 1b/PVA film (1 wt%) only required 150 s at 373 K. We hypothesized that this phenomenon may be due to the interaction of PVA and 1b in the heating process. We further found that the decline curve ascended with continuous heating at 373 K when bleaching of composite film completely finished. The 1b/PVA (0.5 wt%) and 1b/PVA (2 wt%) composite films showed similar phenomenon as shown in Fig. S2 in Supporting information. This may be due to the ascension of the kinetic curves of pure PVA film at different temperatures, as shown in Fig. S3 in Supporting information. Thus the best thermal bleaching temperature for pure
1b and 1b/PVA composite films were 393 and 368 K, respectively. The reversible properties of 1a and 1a in PVA films were further investigated by alternating irradiation with 365 nm and heating for 10 cycles (Fig. 5b,d and Fig. S2). The results indicate that the photochromism of 1a and 1a in PVA matrix exhibit higher fatigue resistance and sensitivity.
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3.4. Photochromism 1a/PVA films for rewritable information storage
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The 1a/PVA composite films are suitable material for optical information recording. As can be seen from Fig. 6, the practical
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Fig. 6. Photographs of 1a/PVA (1 wt%) film before (a, c) and after (b, d) UV irradiation for 2 h. The image d was covered by a mask of ‘‘V’’ during UV exposure.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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optical images of rewritable photoimaging on 1a/PVA composite film are observed. Upon UV light irradiation, the film changes its color from white to yellow, and through a mask, the letter ‘‘V’’ is recorded on 1a/PVA film at irradiation region. Heating the composite films, the optical data are erased. The cycles of writing and erasing are repeated more than 10 times. This successful demonstration of rewritable photoimage suggests that the 1a/PVA composite films can be potentially used in rewritable optical memory media or imaging processes.
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4. Conclusion
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Novel photochromic composite films were successfully prepared based on the pyrazolone derivative 1a embedded in the PVA matrix without any structural alteration by a facile and simple casting technique. The surfactant PVA has been confirmed to be responsible for the good distribution of 1a in composite films. The mechanism of photochromic films resulted from the tautomerization between the enol and keto forms, which is the same to that of pure 1a in crystalline state. The composite films exhibit excellent fatigue resistance under alternating UV irradiation and heating. PVA polymer film provides good thermal conductivity, high mechanical strength and transparency for constructing composite photochromic films. This study opens a new window to fabricate rewritable optical memory media, optical switches, imaging processes using photochromic pyrazolone derivatives.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (Nos. 21262038, 21571152 and U1203292), National 973 Program on Key Basic Research Project of China (No. 2014CB660805), the Outstanding Youth Natural Science Foundation of Xinjiang Uygur Autonomous Region of China (No. 201311006).
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.06. 037.
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Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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CCLET 3760 1–5 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet 1 2 3 4 5 6 7
Original article
Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films Q1 Ji-Xi
Guo, Ming-Xi Guo, Dian-Zeng Jia *, Yin-Hua Li
Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 May 2016 Received in revised form 7 June 2016 Accepted 21 June 2016 Available online xxx
Novel photochromic composite films have been successfully fabricated by dispersing pyrazolone derivative: 1,3-diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenylsemicarbazone (1a) into hydrosol of polyvinyl alcohol (PVA). The microstructure, photochromic behaviors and thermal bleaching properties were investigated by Raman spectroscopy, X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and ultraviolet–visible absorption spectroscopy (UV–vis). The results showed that 1a was not only blended but also well dispersed in the PVA polymer films with a suitable content of chromophore. Upon UV light irradiation, the composite films gradually changed from colorless to yellow and recovered fully to the initial state upon thermal bleaching. The time constants of photochromic reactions were almost the same as those of 1a observed in their crystalline state, indicating that the photochromic phenomenon is barely disturbed by the polymer matrix. ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Pyrazolone Polyvinyl alcohol Composite film Photochromism Information storage
8 9
1. Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Photochromic materials have attracted considerable attention due to their special structures, properties and various applications [1–5]. Pyrazolone derivatives, a new class of organic photochromic materials, have been proven to be promising and important photochromic compounds due to their high sensitivity and excellent fatigue resistance in the solid state [6–9]. Photochromic materials, which display photochromic reactions in the solid state, are fundamental in developing their applications, such as optical information storage [10–12], optical switches [13–16], anticounterfeiting [17,18] and ultraviolet protection [19]. However, in most cases, the photochromic molecules need to be embedded in complex environments such as bulk polymer materials or thin films in order to translate the molecular response into the desired macroscopic effects [20,21]. Therefore, the functional composite films [22,23], which possess reversible photochromism with distinguished color changes, have become a hot topic in material research [24–26]. Pyrazolone derivatives showed no photochromic reaction in solution and amorphous state for their special
* Corresponding author. E-mail address: [email protected] (D.-Z. Jia).
mechanism with inter- and intramolecular proton transfer [27] and common methods for preparing composite films by dissolving chromophores in solvents are not suitable for this system. Therefore, developing of composite films containing photochromic pyrazolones will be a challenge for extending their application. In previous studies, we have reported a composite film based on dispersing the pyrazolone powders in hydroxypropyl methyl cellulose (HPMC) [27]. The composite films showed a reversible photochromic reaction under UV irradiation and thermal bleaching by heating. Unfortunately, the transparency of the composite films is poor. In this work, we have successfully fabricated a thin composite film with 1,3-diphenyl-4-(3-chlorobenzal)-5-hydro-xypyrazole 4phenylse-micarbazone (1a) and polyvinyl alcohol (PVA) in aqueous solution to avoid inactivation of pyrazolones, as shown in Scheme 1. The choice of PVA is vital for the preparation of the composite films not only due to its good film-forming ability, optical transparency and water solubility, but also due to its surfactant effect that enables the distribution of 1a in water solutions. The fabricated composite films exhibited reversible photoisomerization reactions under alternating UV light irradiation and heating as the same as pure 1a observed in the crystalline state. The optical contrast can be easily modified by changing the composite ratio of 1a and PVA. Moreover, the color of 1a in film can be easier tuned by UV stimuli and fades faster than that of 1a in the crystalline state.
http://dx.doi.org/10.1016/j.cclet.2016.06.037 1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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CCLET 3760 1–5 J.-X. Guo et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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Scheme 1. Photochronmic reaction of pyrazolone 1, chemical structure of PVA and photochromic composite film.
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2. Experimental
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The powder X-ray diffraction (XRD) data of the material were collected on a Bruker D8 Advance X-ray diffractometer. Raman spectra were recorded on a Bruker Senterra (R200-L) raman spectrometer. The field emission scanning electron microscopy (FE-SEM) images were obtained on a Hitachi S-4800 scanning electron microscope. UV–visible absorption spectra were studied using a Hitachi U-3010 spectrometer equipped with an integrating sphere accessory and temperature-controlled accessory. A 15 W lamp in ZF-8 Ultraviolet Analysis Instrument with 365 nm excitation light (1.5 W cm 2 on the sample) and a TECH XT-5 melting point apparatus were used as sources for photocoloration and thermal bleaching, respectively. The distance between the sample and light source was 15 cm. Materials: 1,3-Diphenyl-5-pyrazolone was synthesized according to the literature method [28], 3-chlorobenzoylchloride and 4-phenylsemicarbazide were purchased from the J&K Company. PVA powders were purchased from the Aladdin Company. Other materials were purchased from commercial sources, and the solvents were purified with standard procedures. Synthesis of photochromic pyrazolone derivative: 1,3-Diphenyl-4-(3-chlorobenzal)-5-hydroxypyrazole 4-phenyl-semicarbazone (1a) was synthesized through reaction of 1,3-diphenyl-4(3-chlorobenzal)-5-hydroxypyrazole [29] and 4-phenyl-semicarbazone in the presence of acetic acid in ethanol at 80 8C according to the literature [30]. Preparation of 1a/PVA composite films: The 1a/PVA composite films were prepared by the solvent evaporation method. PVA powders (1.5 g) were added to deionized water (10 mL) with vigorous stirring at 80 8C, which provided a viscous mixture. In dark room, the powder 1a (0.015 g) was slowly added to the above solution with continuous stirring for 3 h at room temperature and then left in the dark until the bubbles disappeared. The mixed solution, 1a in the PVA hydrosol with content of 1 wt%, was cast on a smooth glass groove and dried at ambient temperature in the dark for 10 h. Then the composite film was peeled off from the glass substrates. The fabricated composite film (1 wt%) was placed in the dark to prevent the occurrence of photochemical reactions. For comparison, a pure PVA film without 1a addition and 0.5 wt%, 2 wt% composite films were also fabricated following the same procedure. Of note, powders of 1a cannot be dissolved but only dispersed in water.
Fig. 1. Raman spectra of (a) pure PVA film, (b) pure 1a and the different contents of 1a in PVA film (c: 0.5 wt%, d: 1 wt%, e: 2 wt%).
3. Results and discussion
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3.1. Morphology and structure of 1a/PVA composite films
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Raman spectroscopy is a useful tool in the characterization of successful incorporation of 1a into the polymer matrix. The Raman spectra of the pure PVA film, pure 1a and different contents of 1a in PVA film are shown in Fig. 1. It can be clearly seen that the 1a/PVA composite film exhibits the characteristic vibration peaks of 1a at 1605, 1582 and 1000 cm 1, and these peaks are enhanced with increased contents of 1a in film. Meanwhile, the characteristic bands of PVA also appeared to verify the presence of PVA in the composite film. These results demonstrate that the 1a powders dispersed in the polymer films without any structural alteration.
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Fig. 2. XRD of (a) pure PVA film, (b) pure 1a and the different contents of 1a in PVA film (c: 0.5 wt%, d: 1 wt%, e: 2 wt%).
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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Fig. 3. FE-SEM of (a) pure PVA film, insert: pure 1a, and the different contents of 1a in PVA film (b: 0.5 wt%, c: 1 wt%, d: 2 wt%).
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The morphology of materials was characterized by XRD. From Fig. 2, we found that the powders of 1a exhibited the characteristic diffraction peaks at 5.9o, 15.6o, 17.2o, 19.5o and 35.3o (2u), respectively. The lines were sharpened, which demonstrated that the powders of 1a have highly crystalline structures. But the pure PVA film showed an amorphous morphology and presented a broad diffraction peak at 19.7o (2u). We further found that the characteristic diffraction peaks at 5.9o (2u) of 1a in composite films were enhanced with increased contents of 1a in films, and the broad diffraction peak at 19.7o (2u) of PVA still existed in the composite film. These results also provided strong evidence for well dispersing of 1a in the PVA film. The state of 1a in polymer film was further evidenced by FESEM. Based on the above analysis, we know that 1a powders were dispersed into the polymer matrix without any structural alteration. As shown in Fig. 3, the surface of the pure PVA film was very smooth. However, the surface of composite films with different contents of 1a dispersed in PVA matrix became rougher, which showed that rod of 1a was embedded and dispersed in the polymer matrix.
3.2. Photochromic reactions of 1a/PVA composite films
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The powders of 1a can undergo a reversible photochromism reaction upon the alternating treatments of irradiation with UV and heating accompanying with color changes between white and yellow [30]. The absorption spectra of 1a in the crystalline state induced by photoirradiation at room temperature are shown in Fig. 4a. However, the pure PVA film does not respond to UV light. The composite films with different contents of 1a show similar photochromic phenomenon compared with that of the 1a powders (Fig. 4b, c, d). Upon irradiation with 365 nm light, new broad bands in the range of 350–520 nm appeared for composite films with different contents of 1a and their intensities increased with prolonging UV light irradiation. The maximum absorption wavelength of these composite films with different contents of 1a in the PVA matrix was observed at 410 nm, and the ratios of absorbance contrast between the photo-stationary state and initial state were about 3:1 (0.5 wt%), 4:1 (1 wt%) and 5:1 (2 wt%), respectively. These differences are attributed to the different contents of 1a embedded in PVA matrix. The photochromic
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Fig. 4. UV–vis spectra of 1a/PVA composite films (a: pure 1a, b: 0.5 wt%, c: 1 wt%, d: 2 wt%) with different irradiation times at room temperature.
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Fig. 5. Fading kinetics of 1b (a) and 1b (1 wt%) in PVA film (c) at different temperatures. (b) and (d) photocoloration-thermalbleaching cycles upon alternating irradiation with UV light and heating for 1a and 1a in PVA film, respectively.
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mechanism of 1a embedded in PVA film is the same as that of 1a in powders. We further investigated the kinetic of 1a powders and the different contents of 1a in PVA films by monitoring their maximum absorbance changes as a function of irradiation time (Fig. S1 in Supporting information). The first-order kinetics was observed by fitting the experimental data to the equation -kt = 1n[(A-A0)/(AAt)], and the rate constants of 1a and 1a in the PVA matrix obtained from the slope were k1a = 0.0277 min 1, k0.5 wt% = 0.0342 min 1, k1 1 and k2 wt% = 0.026 min 1, respectively. We wt% = 0.0285 min found that the composite films with different contents of 1a in the PVA matrix had different coloration rates, the coloration rate turning slower with increased content of 1a powders.
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3.3. Thermal bleaching reactivity
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In order to understand the impact of microenvironment on the composite film, the thermal bleaching of the photochromic compound 1b and 1b in PVA matrix were investigated at different temperatures (Fig. 5a,c). It was found that 1b and 1b/PVA (1 wt%) film could fade at a high speed at elevated temperatures. But the fading speed of composite films was faster than that of 1b at the same temperature. Sample 1b needed 900 s to be completely bleached; however, 1b/PVA film (1 wt%) only required 150 s at 373 K. We hypothesized that this phenomenon may be due to the interaction of PVA and 1b in the heating process. We further found that the decline curve ascended with continuous heating at 373 K when bleaching of composite film completely finished. The 1b/PVA (0.5 wt%) and 1b/PVA (2 wt%) composite films showed similar phenomenon as shown in Fig. S2 in Supporting information. This may be due to the ascension of the kinetic curves of pure PVA film at different temperatures, as shown in Fig. S3 in Supporting information. Thus the best thermal bleaching temperature for pure
1b and 1b/PVA composite films were 393 and 368 K, respectively. The reversible properties of 1a and 1a in PVA films were further investigated by alternating irradiation with 365 nm and heating for 10 cycles (Fig. 5b,d and Fig. S2). The results indicate that the photochromism of 1a and 1a in PVA matrix exhibit higher fatigue resistance and sensitivity.
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3.4. Photochromism 1a/PVA films for rewritable information storage
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The 1a/PVA composite films are suitable material for optical information recording. As can be seen from Fig. 6, the practical
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Fig. 6. Photographs of 1a/PVA (1 wt%) film before (a, c) and after (b, d) UV irradiation for 2 h. The image d was covered by a mask of ‘‘V’’ during UV exposure.
Please cite this article in press as: J.-X. Guo, et al., Preparation and photochromic properties of pyrazolones/polyvinyl alcohol composite films, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.06.037
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optical images of rewritable photoimaging on 1a/PVA composite film are observed. Upon UV light irradiation, the film changes its color from white to yellow, and through a mask, the letter ‘‘V’’ is recorded on 1a/PVA film at irradiation region. Heating the composite films, the optical data are erased. The cycles of writing and erasing are repeated more than 10 times. This successful demonstration of rewritable photoimage suggests that the 1a/PVA composite films can be potentially used in rewritable optical memory media or imaging processes.
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4. Conclusion
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Novel photochromic composite films were successfully prepared based on the pyrazolone derivative 1a embedded in the PVA matrix without any structural alteration by a facile and simple casting technique. The surfactant PVA has been confirmed to be responsible for the good distribution of 1a in composite films. The mechanism of photochromic films resulted from the tautomerization between the enol and keto forms, which is the same to that of pure 1a in crystalline state. The composite films exhibit excellent fatigue resistance under alternating UV irradiation and heating. PVA polymer film provides good thermal conductivity, high mechanical strength and transparency for constructing composite photochromic films. This study opens a new window to fabricate rewritable optical memory media, optical switches, imaging processes using photochromic pyrazolone derivatives.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (Nos. 21262038, 21571152 and U1203292), National 973 Program on Key Basic Research Project of China (No. 2014CB660805), the Outstanding Youth Natural Science Foundation of Xinjiang Uygur Autonomous Region of China (No. 201311006).
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.06. 037.
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